There are known many alternate interconnected suspension systems which have the ability to passively differentiate between different modes of wheel motion with respect to the vehicle body and therefore provide a variety of alternatives in functionality. For example, French patent publication number FR 2 663 267 discloses a hydro-pneumatic suspension system which supports the vehicle body and provides different heave and roll stiffness rates with zero warp stiffness. The pitch stiffness is related to the heave stiffness (which is determined by the rod diameters, accumulator sizes and ram positions front and rear). Since this system supports the vehicle, the pressures in the four fluid volumes must be high, or the rod diameters large to provide the support force for the vehicle, which leads to seal friction and significant ride comfort limitations. Furthermore, as the temperature changes, the volumes of gas and fluid in the system cause ride height changes, requiring a costly high pressure fluid supply and control system. Also, the pressure in each of the four fluid volumes must be individually correct to provide the required support at each wheel, so the pressures are usually different, leading to control complexity, leakage past the piston seals between the fluid volumes and increase in piston seal friction.
Similarly, the applicant's U.S. Pat. No. 6,270,098 provides a pressure balancing “load distribution” unit between two pairs of diagonally interconnected double acting wheel rams. This system provides different heave, roll and additionally pitch stiffness rates with zero warp stiffness and different damping rates in all four base suspension modes (heave, roll pitch and warp). Since this system also supports the weight of the vehicle, as the loads on the vehicle change, or as the fluid temperature changes, the volume of fluid in each of the six volumes in the system must be adjusted. Also, as the six volumes in the system can, in some load conditions, all be at different pressures, there is the possibility for fluid to leak across seals, which also requires fluid volume adjustments to be made to maintain the correct vehicle attitude. This requires a high pressure fluid source, sensors, control electronics and valves, making the cost of the system relatively high for a passive system.
Likewise in EP 1 426 212 and International Application Number PCT/EP2004/004885 there are disclosed a number of passive hydraulic systems providing support of the vehicle and roll stiffness with zero warp stiffness. As these hydraulic systems provide support of the vehicle they have similar disadvantages to the applicant's aforementioned U.S. Pat. No. 6,270,098.
An example of a passive system providing high roll stiffness with low warp stiffness and negligible heave stiffness and providing high roll damping with lower, more comfortable and isolating heave damping can be found in the applicant's U.S. Pat. No. 6,761,371. As the system does not provide significant heave stiffness, separate support springs are required. The roll moment distribution required of the hydraulic system drives the selection of wheel ram sizes and can lead to compromises in peak damping force.
An example of a system having just roll and/or pitch damping can be found in U.S. Pat. Nos. 5,486,018 and 6,024,366. The system in these documents uses a device between a pair of wheel damping rams, each wheel damping ram having a damper valve in its piston to provide double-acting damping but make the ram single-acting (i.e. there is only a single fluid port). The device provides for independent levels of damping for in-phase (i.e. heave) and out of phase (i.e. roll and/or pitch) motions. However this system does not provide significant stiffness in any mode, so in addition to the need for support springs, generally anti-roll bars will be required for a good balance between bounce and roll stiffness rates front and rear. Additionally, as the wheel rams are effectively single acting (having only one fluid port) the amount of damping that the device can provide is limited. There are improvements made to the system to combat this problem, which can be found in Japanese patent office publication number 11291737, but these add to the complexity of the system by providing more plumbing and spool valves.
It is therefore an object of the present invention to provide a hydraulic system for a vehicle suspension that alleviates at least one of the disadvantages of earlier vehicle suspension systems.
It is a preferred object of the present invention to provide a hydraulic system having roll stiffness, roll damping, and heave damping, the roll moment distribution of which can be configured and tuned substantially independently from the wheel damping pressures.